This invention relates to electronic semiconductor devices, and particularly to a packaged device structure which includes a chip on a metal plate, surrounded by a plastic body from which electrical leads protrude.
As is known, integrated circuits and other active electronic devices or components.sup.1 are typically formed on small chips (or "dice") of a monocrystalline semiconductor material. Typically these chips have a surface area on the order of a few square millimeters, and require for their connection to an external electric circuit special supporting, enclosing and electric interconnection structures. FNT .sup.1 The term "integrated circuits" usually refers to devices which include many transistors interconnected to provide analog or digital or mixed circuits. However, many of the packaging requirements of integrated circuits also apply to other semiconductor devices, such as power transistors or thyristors, discrete diodes or sensor devices, optoelectronic devices, etc.
A typical structure suited for the purpose comprises basically a plastics body enclosing a chip which is connected, by thin wires soldered to metallized regions specially provided on its surface, to corresponding electric conductors led out of the plastics body. With power integrated circuits (which operate at high currents, and therefore must dissipate a relatively large amount of heat), such structures also comprise a small metal plate through which a chip mounted thereon can transfer the heat generated during its operation to the environment.
In manufacturing such power device packages, a chip is secured on the metal plate, either by soldering with a low-melt alloy (such as a lead-tin alloy), or by cementing with a suitable adhesive such as a conductive or nonconductive epoxy adhesive. A "lead frame" 16 (i.e. a set of metal strips 11 still left joined to one another by interconnecting sections) is mounted to the metal plate in an electrically insulated manner therefrom. (In the presently preferred embodiment, this insulation is actually provided by the resin body 10, but of course alternatives can be used instead. During assembly the strips are kept in position by the interconnecting portions 15 and the beams 19 fixed to the plate 12.)
Thin wires 18 (of gold in the presently preferred embodiment) are soldered, on the one side, to the metallized regions of the chip using a low-melt alloy, and on the other side, welded to the ends of the metal strips using a so-called "thermosonic" process (wherein heat and ultrasound are applied simultaneously). Thereafter, the assembly is placed into a specially provided mold, into which a resin material such as a thermosetting epoxy resin is then introduced in a liquefied state to surround the chip and the bond wires. This resin is then cured to produce a solid plastic body which totally encapsulates the above-described elements, except for part of the metal plate and part of the device terminal leads. The outer portions of the lead frame, which have held the leads in place, are then removed (e.g. by blanking) to yield the finished electronic product, with multiple electrically separate leads exposed.
It is necessary in some cases that at least one of the metallized regions on the chip be connected electrically to the metal plate. According to a prior art technique, this connection is established by a thin wire, usually made of gold: one end is attached to the metallized region of the chip, and the other end is attached, by ultrasonic thermal welding, to the same surface of the plate where the chip is mounted. However, this type of connection is not as reliable as it should be; it has been found, in particular, that the wire tends to separate from the plate due to failure of the welded joint. A cause of this problem has been found to be contamination of the plate surface by the chip attachment material, whether a soldering alloy or an adhesive, so that quality of the ultrasonic thermal weld becomes low and likely to fail under mechanical or electrical stresses, however slight.
To remove such causes for failure, it has been proposed to form a groove around the plate area where the chip is to be affixed. The groove provides a barrier to limit the outflow of molten solder or liquefied adhesive across the plate, thereby lowering the risk of the surface being contaminated.
It has been found that, despite this precautionary measure, the faulty pieces due to the connecting wire to the plate coming loose are still excessive.
The disclosed innovations provide an electronic device structure which is free from the aforementioned drawback. This object is achieved by a structure which includes a shaped metal connection (e.g. a swaged joint) between widened portion of the lead frame and a metal backing plate to which the semiconductor chip is attached. Instead of the usual ground wire routing, ground wires are connected from the chip to the widened portions of the lead frame.